FIG. 15 is a circuit diagram illustrating an electrical discharge machining power supply which is representative of the power supply circuit disclosed in Japanese Patent Publication No. 60-123218. As shown in FIG. 15, the circuit includes a direct-current power supply circuit 1 including a direct-current power supply 2 and a capacitor 3. It is desirable for the capacitor 3 to be a rapid-response capacitor and for the direct-current power supply circuit 1 to be a circuit having a small inductance. The circuit of FIG. 15 further includes an electrical discharge machining electrode 4, a workpiece 5, rectifying devices 8, 9 (e.g., diodes) and MOSFETs 6 and 7 serving as switching devices and including parasitic diodes, respectively. The MOSFET 6 and the rectifying device 8 are connected in series with the direct-current power supply circuit 1 to form a first series circuit, and the MOSFET 7 and the rectifying device 9 (which are opposite in connection sequence to the first series circuit), are also connected in series with the direct-current power supply circuit 1 to form a second series circuit. The electrical discharge machining electrode 4 and the workpiece 5 are in series with each other between connection points X and Y. Connection point X is a connection point between the MOSFET 6 and the rectifying device 8 in the first series circuit, and connection point Y is a connection point between the MOSFET 7 and the rectifying device 9 in the second series circuit. A closed loop including the direct-current power supply 2, the MOSFET 6, the workpiece 5, the electrical discharge machining electrode 4 and the MOSFET 7 constitutes a discharge circuit, and a closed loop including the capacitor 3, the rectifying device 8, the workpiece 5, the electrical discharge machining electrode 4 and the rectifying device 9 constitutes a feedback circuit. The reference numerals 10 and 11 represent wiring inductances in the circuit.
The operation of the circuit shown in FIG. 15 will now be described with reference to the timing chart shown in FIG. 16. FIG. 16(a) shows a repeated pulse applied to the gates of the MOSFETs 6, 7, the application of which causes the MOSFETs 6, 7 to alternately turn on and off in synchronization with each other. FIG. 16(b) shows a current Ids flowing in the MOSFETs 6, 7. FIG. 16(c) shows a current Iak flowing in the rectifying devices 8, 9. FIG. 16(d) shows a current IG flowing from point X to Y in FIG. 15, i.e. the current IG flowing in the wiring inductances 10, 11 and in a gap between the electrical discharge machining electrode 4 and the workpiece 5. FIG. 16(e) shows a voltage Vxy across the connection points X and Y in FIG. 15, and FIG. 16(f) shows a voltage VG imposed across the gap.
When the pulse shown in FIG. 16(a) is input to the gates of the MOSFETs 6, 7, and the MOSFETs 6, 7 are switched on simultaneously, the voltage VG is applied to the gap between the electrical discharge machining electrode 4 and the workpiece 5 to start a discharge, and the gap current IG flows in the gap. Specifically, the current flows from a power supply terminal A to a power terminal B via the MOSFET 6, the workpiece 5, the electrical discharge machining electrode 4 and the MOSFET 7 (which constitute the discharge circuit). If Vo is the terminal-to-terminal voltage of the direct-current power supply circuit 1, and L1 and L2 are the inductances of the wiring inductances 10, 11, respectively, then the following relationship exists for the gap circuit Ig: EQU dIg/dt=(Vo-Vg)/(L1+L2)
Hence, the current increases linearly as shown in FIGS. 16(b) and (d). When the MOSFETs 6, 7 are switched off, however, the current accumulated in the wiring inductances 10, 11 flows through the feedback circuit. That is, the current flows from the power supply terminal B to the capacitor 3 of the direct-current power supply circuit 1 via the rectifying device 8, the workpiece 5, the electrical discharge machining electrode 4, the rectifying device 9 and the power supply terminal A. Since the direction of the current flow and the direct-current power supply circuit 1 are opposite in polarity, the current reduces linearly according to the following relationship: EQU dIg/dt=(-Vo-Vg)/(L1+L2)
(See FIGS. 16(c) and (d)).
This reduction in the current continues for a certain amount of time, and the current may drop below zero amperes due to the reverse recovery characteristics of the rectifying devices 8, 9. The current then returns to zero at a large slope determined by the rectifying devices 8, 9, and the rectifying devices 8, 9 are switched off. At this time, as shown in FIG. 16(e), the voltage Vxy across the connection points X and Y in FIG. 15 becomes an excessive voltage which is determined by the following relationship: EQU Vxy=[(L1+L2)] diD/dt+Vg
The excessive voltage is due to the large slope diD/dt which is determined by the rectifying devices 8, 9.
If, for example, the output voltage Vo of the direct-current power supply circuit 1 is 220V, the gap voltage Vg is 20V, the wiring inductance is 3.mu.H, and the ON time of MOSFETs 6, 7 is 1.5.mu.s, then the current becomes: EQU [(220V-20V)/3.mu.H] 1.5.mu.s=100A
Accordingly, it is possible to cause a large peak current of 100A, with a current pulse width at this of approximately 3.mu.s (see FIG. 16(d)). However, if it is assumed that the reverse recovery time of the rectifying devices 8, 9 is 90ns, and that the current keeps decreasing for the first 60ns according to a slope determined by the wiring inductances 10, 11 and returns to zero amperes in the remaining 30ns, a reverse recovery current Irp (the maximum instantaneous value of the reverse current) is: EQU Irp=[(220V-20V)/3.mu.H] 60ns=4A;
and a voltage Vlp developed in the wiring inductances 10, 11 at a time when the reverse recovery current Irp returns to zero amperes in the remaining 30 ns is: EQU Vlp=3.mu.H.times.4A/30ns=400V.
In the electrical discharge machining power supply circuit shown in FIG. 15, an excessive voltage generated in the wiring inductances 10, 11 may cause the MOSFETs 6, 7 to be damaged when the MOSFETs 6, 7 are switched off and the current accumulated in the wiring inductances 10, 11 decreases. This is because the rectifying devices, even if of an identical type, may vary in reverse recovery time. For example, if at the instant that the rectifying device 8 turns off the rectifying device 9 still remains in a reverse recovery state, then the potential at the connection point Y in FIG. 15 will almost be equal to that of the terminal A because the rectifying device 9 is on, and the potential at the connection point X in FIG. 15 will be the sum of the potential at the point Y and the excessive voltage because the rectifying device 8 is off. At this time, a current flows through the parasitic diode of the MOSFET 6. Specifically, the current circulates from the rectifying device 9 to the parasitic diode of the MOSFET 6 via the electrical discharge machining electrode 4 and the workpiece 5. Since the parasitic diode of the MOSFET 6 has a relatively small current capacity, it is likely that the MOSFET 6 will be damaged if this circulation occurs one or more times.